Vinylidene fluoride resin is excellent in chemical resistance, heat resistance and mechanical strength and, therefore, has been studied with respect to application thereof to porous membranes for separation. Many proposals have been made regarding porous membranes of vinylidene fluoride resin, for water (filtration) treatment, particularly for production of potable water or sewage treatment, and also processes for production thereof (e.g., Patent documents 1-6 listed below).
Also, the present inventors, et al., have found that a process of melt-extruding a vinylidene fluoride resin having a specific molecular weight characteristic together with a plasticizer and a good solvent for the vinylidene fluoride resin into a hollow fiber-form and then removing the plasticizer by extraction to render the hollow fiber porous is effective for formation of a porous membrane of vinylidene fluoride resin having minute pores of appropriate size and distribution and also excellent in mechanical strength, and have made a series of proposals (Patent documents 7-10 and others). However, a strong demand exists for further improvements of overall performances including filtration performances and mechanical performances of the porous membrane necessary for use as a filtration membrane. For example, as an MF (microfiltration) membrane used for the purpose of, e.g., production of potable water or industrial water by clarification of river water, etc., or clarification of sewage, it is required to have an average pore size of at most 0.25 μM for secure removal of Cryptosporidium as a typical injurious microorganism, and causes little contamination (clogging) with organic substances on the occasion of continuous filtration operation of cloudy water, to maintain a high water permeation rate. From this viewpoint, a porous membrane proposed by Patent document 6 below has an excessively large average pore size, and a hollow-fiber porous membrane proposed by Patent document 8 retains a problem in maintenance of a water permeation rate in continuous filtration operation of cloudy water.                [Patent document 1] JP-A 63-296939        [Patent document 2] JP-A 63-296940        [Patent document 3] JP-A 3-215535        [Patent document 4] JP-A 7-173323        [Patent document 5] WO01/28667A        [Patent document 6] WO02/070115A        [Patent document 7] WO2005/099879A        [Patent document 8] WO2007/010832A        [Patent document 9] WO2008/117740A        [Patent document 10] WO2010/082437A        